Description of the Prior Art
The present invention relates to a valve rotating apparatus and more particularly to a valve rotating apparatus for the intake/exhaust valve of a four-cycle engine.
The intake/exhaust valve of a four-cycle engine is attached, as biased in the closing direction, to a valve seat disposed at the engine head by a coil spring. The intake/exhaust valve is adapted to intake and exhaust, respectively, as opened and closed by a cam formed at a cam shaft (or a rocker arm) through a valve lifter biased by the spring. However, the valve is often repeatedly opened and closed with respect to the valve seat. This may cause the contact surface of the valve to be unevenly worn. Such uneven wear of the valve makes it difficult to hermetically seal the inside of the engine, provoking a variety of troubles. Accordingly, various proposals for rotating the valve have been conventionally made.
As an example of the prior art for rotating the valve, there is known a valve operating mechanism disclosed in the specification of U.S. Pat. No. 4,597,408. This valve operating mechanism comprises: a poppet valve (5) having a threaded end of its stem (10); a valve spring retainer (14) having a center bore for the valve stem, a circular groove for a coil spring (12) and a first ball bearing (18) race on the opposite face from the circular groove; and a valve retainer (16) having a threaded bore and a second ball bearing race. A set of ball bearings is placed at one face of the spring retainer (14). The valve stem (10) is inserted through through the spring (12) and the spring retainer (14), and the valve retainer (16) is threadedly connected to the screw at the stem end. This causes the ball bearings (18) to be fitted to the valve stem. In the ball bearing having the arrangement above-mentioned, the poppet valve (5) is adapted to be rotated by the operation of the spring and the spring retainer.
Japanese Utility-Model Laid-Open Publication No. 160707/1987 discloses a valve operating mechanism of an internal-combustion engine. This valve operating mechanism has a valve spring (19) for biasing an end of a valve (1) attached to a cylinder head (3) through a valve guide (5), toward the cylinder head in the closing direction, and is characterized in that the spring seat of the valve spring (19) at the cylinder head (3) side is formed integrally with the valve guide and that the valve guide is rotatably disposed to the cylinder head.
Japanese Utility-Model Laid-Open Publication No. 190810/1987 discloses a valve operating mechanism of an internal-combustion engine. This valve operating mechanism comprises a valve to be opened and closed by a rocker arm, a coil spring for biasing the valve toward the cylinder head in the closing direction, a retainer disposed between the valve and the coil spring and secured to the stem end of the valve and a needle bearing disposed between the retainer and the coil spring, and is characterized in that the needle bearing is formed by a plurality of needle rollers and a case for holding the needle rollers therein, and that the case has slits through which portions of the needle rollers project from the case.
Except for the so-called overhead- or side-valve type, any of the prior art above-mentioned is arranged such that the center line of the cam is not aligned with the center line of the valve lifter, thus positively rotating the intake/exhaust valve at its surface coming in contact with the valve seat. This prevents the valve from being unevenly worn.
The prior art above-mentioned presents no problem in view of the rotation direction, but has no degree of freedom for movement in the radial direction. More specifically, when the coil spring is compressed by the drive force of the cam, the coil spring is not always compressed accurately and may be compressed eccentrically with respect to the center line of the spring if the mounting precision is insufficient or if the spring is jumped. This presents the problems that what the coil spring is bent, a radial component is applied to the retainer of the coil spring and the roller is unevenly worn. In the type where a ball bearing is used, the balls may be unevenly raised to the bearing ring. There are also instances where a low-friction thrust washer or the like is used as the retainer. However, when the center line of the valve is not aligned with the center line of the cam, the rotating-driving force is not great. Accordingly, the valve is not rotated at a low rotational speed, and is rotated only at a high rotational speed.